I filled the bottom of mine to the waist with plaster, the arms with newspaper, then filled the top to the neck hole with expanding foam. Expanding foam expands alot. A little goes a long way. If you try this, squirt a bit in, let it sit for a couple hours, then if needed squirt a bit more, and wait again.
It's better to take you time than to ruin the kit by squirting a tons of the stuff in there and have it expand right out of the neck hole and all over the kit. That would be a mess.
Also there will probably be some big gaps due to the malformation of some parts. I used MagicSculp epoxy putty to fill the gaps and blend it into the skin.
Good luck, this is a great kit.
Brian
Here are some pics of mine.
http://img.photobucket.com/albums/v247/12Rogues/On%20the%20bench/Finished%20Kits/Fin-1.jpg
http://img.photobucket.com/albums/v247/12Rogues/On%20the%20bench/Finished%20Kits/fin-2.jpg
http://img.photobucket.com/albums/v247/12Rogues/On%20the%20bench/Finished%20Kits/fin-3.jpg